In the morning after cleansing and toning, apply 4-5 drops to a dry face, neck, and chest before other skincare products. May be used on the back of the hands. It is normal for vitamin C products to darken after opening due to air and light exposure. The formula will remain effective. Once opened, it is recommended to be used within 3 months of opening. Complete your regimen with a SkinCeuticals sunscreen. When used together, SkinCeuticals antioxidants and broad spectrum sunscreens provide the most complete protection against visible aging caused by environmental aggressors.